1. Field of the Invention
The present invention relates to an image forming apparatus by an electro-photography process such as a copying machine, a printer, a facsimile machine or a combined machine of these, the apparatus having a means for restricting a distance between a photosensitive body drum and developing roll or a distance between a developing roll and a magnetic roll and more particularly to a developing unit used in an image forming apparatus such as a copying machine, a printer, a facsimile machine or a combined machine of these. Further the present invention relates more particularly to an image forming apparatus having a developing unit that develops an electrostatic latent image by placing only charged toner on a developing roll from a two component developer material, in which a non-magnetic toner is charged by means of a magnetic carrier, and by causing the toner to jump to the electrostatic latent image.
2. Description of the Related Art
Heretofore, a non-contact system image forming apparatus that develops an electrostatic latent image by placing only charged toner on a developing roll from a two component developer material, in which a non-magnetic toner is charged by means of a magnetic carrier, and by causing the toner to jump to the electrostatic latent image has been publicly known by Japanese laid-open patent publication, JP1994-67546, JP1995-92804 or others. In these prior arts, since an apparatus has a photosensitive body drum, a developing roll and a magnetic roll, the gap between the photosensitive body drum and the developing roll or between the developing roll and the magnetic roll being large so as to affect the image, a structure giving consistently stable distance for the both gaps are required. Meanwhile, Japanese laid-open patent publication, JP2000-275967 (prior art 1) discloses the art in which a spacer roller having a larger diameter than the developing roll is provided coaxially with the developing roll to restrict the distance between the photosensitive body drum and the developing roll.
Further, Japanese laid-open patent publication, JP2001-13848 (prior art 2) discloses the following art that prevent dappled images by reducing and uniformizing a contact pressure between a developing roll and a photosensitive roll. A developing unit is fixed to a photosensitive body unit so as to be capable of swinging and a point of action for communicating rotational force to the developing unit is disposed in the vicinity of swinging center axis of the photosensitive body and the developing unit whereby since when rotational force affects the developing unit, the developing unit swings with respect to the photosensitive body unit, the developing roll can contact the photosensitive roll at the constantly stable state without causing the developing roll to be pressed to the photosensitive body.
In the prior art 1, since the photosensitive roll, the developing roll and a supporting member for the spacer roller are supported in a same developing frame, even though the spacer roller touches the surface of the photosensitive body drum, a weight of the supporting member of those things including the developing roll and the spacer roller does not affect the photosensitive roll. However, in case a structure is constituted as a separate developing unit including a developing roll, a spacer roller, a magnetic roll, a toner container and others, which is attachable to and detachable from the image forming apparatus body, for replenishing toner by pulling out the developing unit, since a weight of the developing unit affects the photosensitive body drum, reduction of durability of the photosensitive body drum is apprehended.
Further, an appropriate gap not only between the developing roll and the photosensitive body drum but also between the developing roll and a magnetic roll is necessary to be kept. Variation of the distance between them causes variation of the layer thickness of magnetic brush on the magnetic roll, which further causes variation of the layer thickness of toner on the developing roll. As these result in unsatisfactory image formation, a complicated structure to ensure toner supply to the developing roll by increasing the rotational rate of the developing roll is necessary.
In the prior art 2, though the art in which a developing roll touches a photosensitive body in a constantly stable state by the means that a developing unit is fixed to a photosensitive body unit so as to be capable of swinging is disclosed, the art is merely for stabilizing the touch between the developing roll and the photosensitive body drum. In JP1994-67546, JP1995-92804 or others, the problem is it is difficult to keep a stable distance between the photosensitive body drum and the developing roll or between the developing roll and the magnetic roll in a developing unit using two components developer material.